In case plating is performed on an object in planar shape such as a printed board for example, there are two methods: intermittent carrying method for intermittently carrying the objects to be plated by hanging it vertically by crane, and horizontal carrying method for continuously carrying the objects to be plated into a plating tank by horizontal transport means. The former is also called the batching processing, and the latter the continuous processing. On the other hand, the system is divided into soluble electrode type and insoluble electrode type according to the sturcture of the anode used.
Describing the intermittent carrying type in detail, in a plating equipment of soluble electrode type as shown in FIG. 5, 1 represents a plating tank, 5 a printed board (object to be plated) tank, 5 a printed board (object to be plated) supported by a carrying rod 6, and 21' and 21' are anodes supported by power feeding hangers 2 and 2. This anode 21' comprises a mesh-like cylinder accommodating balls, chips, etc. of copper or copper alloy.
In such plating apparatus, however, troublesome procedure is required for maintenance and control of copper ion concentration in the plating solution because the anode (such as copper ball) 21' is soluble, and much labor and time are required for supply and replacement of the anodes itself. Further, in order to prevent non-uniformity of the plating film thickness caused by the change of interpolar distance due to wearing of anode, the interpolar distance l must be increased. This involves such problems as the large-size equipment or the increase of initial make-up of electrolytic bath.
To solve the above problems, it has been proposed to use isoluble anode and to supply copper ions, generated in a separate copper dissolving tank, to the plating tank. However, in case of such insoluble anode, equilibrium potential is increased compared with a soluble anode, and this enhances oxidizing decomposition of the additives (such as polyether, organic sulfur compound, etc.) by positive polarization. As the result, the consumption of the additives is increased, and this in turn raises the cost of electroplating, resulting in the problems caused by the introduction of insoluble anode.
To ensure stable long-term use of additive by preventing the decomposition of the additives and to facilitate the plating solution control, a method to separate the anode (21) from the object to be plated (5) by a diaphragm has been proposed (e.g. Japanese Provisional Patent Publication No. 297884/1989).
Specifically, as shown in FIG. 6, insoluble plate electrode is used as anode 21, and a diaphragm 31 is provided between the anodes 21 and the object to be plated 5. In such insoluble electrode system, most of the plating solution containing the additive is not brought into direct contact with the anode 21, and this prevents the decomposition of the additive and minimizes the use of expensive reagents.
However, even in this insoluble electrode system, the diaphragm 31 is eaily deformed or broken, and it is difficult to inspect and replace the diaphragm 31. The present applicant previously proposed an integrated type apparatus as shown in FIG. 4, in which an insoluble electrode 21 and a diaphragm 31 are placed in a liquid-tight main unit case 11 and the diaphragm 31 is covered by rib member 41 (Japanese Patent Application No. 215636/1990). In FIG. 4, 15 represents a spacer, and S denotes an accommodation space, i.e. closed space, for the electrode 21. By such arrangement, the diaphragm 31 can be protected perfectly, and inspection can be rapidly and easily performed. Also, uniform current distribution can be assured by the separation effect of the upright planes 42a formed by a plurality of grids 42.
In the batch processing system by intermittent carrying type, it is apparently advantageous for the improvement of productivity and compact design of the equipment to provide a continuous processing system, in which soluble anodes 21' are arranged above and below of a plating tank 1 as shown in FIG. 5 or insoluble anodes 21 and diaphragms 31 are furnished above and below in the plating tank 1 as shown in FIG. 6, and the object to be plated 5 is carried horizontally by a conveyor through the space between them.
However, the continuous processing system of horizontal carrying type is not widely used at present.
The reasons for this may be as follows: In case the soluble anodes 21' are furnished above and below as shown in FIG. 5, non-uniformity of plating film thickness caused by the change of interpolar distance due to wearing of anode as explained in the above batch processing becomes more conspicuous and the replacement of the anode itself is also difficult. On the other hand, in case the insoluble anodes 21 are furnished above and below as shown in FIG. 6, the inspection and replacement of the diaphragms 31 becomes more complicated and difficult.
In this respect, the present applicant has studied and produced a continuous processing system on trial by adopting the above integrated type diaphragm electrode structure (11, 21, 31 and 41) as proposed previously (Japanese Patent Application No. 215636/1990). Although perfect protection can be provided for the diaphragm 31 and the inspection and replacement of the diaphragm 31 are done easier in this system, new problems have risen such as wide variation of plating film thickness due to the stagnation of the gas generated from insoluble anode 21 in the closed space S formed by main unit case 11 and diaphragm 31, and the difficulty for feeding electric current smoothly. Particularly, in case of copper plating on plate type printed board having through-hole, the proposed system cannot be used practically.